Carbon capture and storage (CCS) has been proposed and to some extent adopted as a mid- to long-term measure in the fight to mitigate the undesirable effects of global warming by reducing the emission of greenhouse gases. A significant element that has been missing, particularly in the field of stationary power plants, is a technically feasible and economically sound means for operating a carbon capture system. Several different techniques exist today that can be used to obtain CO2 in a pure stream from a combustion unit. The three most often mentioned are post-treatment, O2/CO2 firing, or oxyfuel, and CO-shift.
One variety of petroleum is heavy oil. Like the so-called “bottom of the barrel” of conventional petroleum, heavy oil is quite carbon-rich and by definition very dense. It is estimated that heavy oil production will increase by 200 percent in 2030. Of the world's total oil reserves, an estimated 53 percent are in the form of heavy oil or bitumen, which terms are used interchangeably to describe oil that is highly viscous, solid or near-solid at room temperature, e.g., tar or asphalt, that has a relatively low hydrogen content and that also has a high mass density, e.g., an API gravity of 20 degrees or less. Vaporization of heavy liquid fuels is not always practical due to the composition and the physical characteristics of these fuels and vaporization of heavy fuel oil can require a complex feeding system that is more costly than a direct liquid fuel oil feeding system.
Refining heavy oil presents a technical challenge. However, as the price of lighter crude oil increases, the economics for upgrading heavy oil to a synthetic crude oil continue to improve. However, it is difficult to use a single technology for upgrading the various types of heavy oil. They have very high viscosity compared to typical liquid fuels and the viscosity changes dramatically with temperature. Vacuum distillation bottoms which flow like water at 300° to 400° C., but remain solid at room temperature are one of the most difficult refinery materials to handle and transport. These heavy oils usually come out of a refinery process in liquid form at elevated temperatures and must be introduced directly into the combustor in this form. Circulating fluidized bed boilers (“CFB”) can burn refinery by-products efficiently and cleanly. The CFB process employs the circulation of fluidized particles.
Chemical looping combustion (“CLC”) is another process that has received increasing attention for more than a decade. Chemical looping combustion can be used for converting hydrocarbon fuels to produce heat energy, e.g., for power generation, and from which process an essentially pure stream of CO2 can be separated and recovered. Chemical looping combustion is of increasing economic interest because of the comparatively low energy loss associated with the overall operation of this process.
The CLC process is based on the principle that an oxygen carrier can serve as an intermediary to transport oxygen between atmospheric air and the fuel without direct contact of the air and the fuel. The primary advantage is the elimination of nitrogen and excess oxygen from the combustion zone and, therefore, from the combustion gases produced. A solid oxygen carrier such as a metal oxide oxidizes the fuel, which results in the production of CO2 and H2O. The reduced form of the oxygen carrier is then transferred to an air reactor where the reduced particles are contacted by atmospheric air and are oxidized to their initial state, and then returned to the combustion chamber or reactor(s). The overall heat of the chemical looping process is the sum of the two heat states, i.e., exothermic during oxidation of the fuel and endothermic during reduction, and the sum is equal to the heat released in a conventional combustion reaction. In the overall context of a carbon capture process, the main advantage of the chemical looping process is that minimum additional energy that is required to effect the CO2 capture. The overall combustion efficiency is comparable to the conventional combustion processes, i.e., there is a minimum energy penalty for CO2 capture that has been estimated to result in only a 2-3% overall efficiency loss. Moreover, NOx formation is reduced in this process, since oxidation occurs in the air reactor in the absence of fuel and at temperatures below the 1200° C. at which NOx formation increases considerably.
The CCS process has the dual advantages of both environmental and economic benefits. For example, the CO2 can be used in other industrial applications after being captured. Enhanced Oil Recovery by injection of CO2 (CO2-EOR) into oil reservoirs to increase petroleum production is one established industrial use for CO2 that has been applied commercially for about 40 years. The CO2 has been derived principally from natural sources, and its feasibility has been recognized in terms of the economics of transporting of the CO2 and its controlled injection into the hydrocarbon reservoir.
Patent publication US 2011/0171588 entitled Optimized Method and Device Loop Combustion on Liquid Hydrocarbon Feedstock describes an apparatus and method for CO2 capture and energy production utilizing the chemical looping combustion of at least one liquid hydrocarbon feed that is mixed with an atomization gas and sprayed into contact with at least a portion of the total metal oxide particles in a riser reactor referred to as a transport zone and at a flow rate that is selected so that the superficial gas velocity of the sprayed liquid feed is greater than the transport velocity of the metal oxide particles. Thereafter, the effluents from the transport zone are introduced into a combustion zone that comprises at least one dense-phase fluidized bed where an additional quantity of metal oxide particles are reduced and combustion of the fuel is completed. The thermal cracking occurs and coke is deposited on the metal oxide particles. In some embodiments, combustion is completed in up to three separate reactors in series to burn the coke from the particles. The process described achieves the main part of the combustion of the coke deposited on the particles in the combustion reactor and the gaseous effluents of the oxidation reactor comprise a maximum of 10 percent of the total CO2 produced during the overall combustion process.
It is known that various metals can be used as the oxygen carrier. Transition metal oxides such as nickel, copper, cobalt, iron and manganese are preferred because of their favorable reductive/oxidative thermodynamic properties.
The selection and design of the reactor can be important to the overall efficiencies of the CLC system. For example, in a riser reactor, e.g., a conventional FCC process of injecting liquid hydrocarbon oil in contact with solid oxide catalysts, a phenomenon referred to as back-mixing can occur which adversely affects the distribution and contact time of the co-currently moving solid and feed. When back-mixing does occur, the distribution of the catalyst/gas retention times throughout the volume of the reactor can vary widely. Additionally, the radial distribution of the catalyst concentration in the reactor is not uniform. The deviation in the speed/concentration distribution within the riser resulting from back-mixing can be especially severe when the flow rate is high, i.e., when the reactor is operated for maximum production to meet the refinery's specification.
The problems addressed by the present invention are how to further improve the overall efficiency of the chemical looping process of the prior art for the capture of an essentially pure stream of CO2, including providing a simpler system with fewer components to thereby reduce the capital costs associated with the construction of the CLC unit and the day-to-day operating and maintenance expenses of the unit.
A related problem addressed is increasing the proportion of CO2 recovered from the combustion step and increasing the amount of energy generated relative to the amount of oxidized oxygen carrier circulated in the system.
Another problem addressed by the present invention is how to construct and operate a CLC system that minimizes both the retention time of the oxygen carrier particles and the amount of oxygen carrier that must be circulated to achieve the required heat energy output, and also avoid the known problems of back-mixing associated with prior art reactors.
As used herein, the terms “metal oxide particles”, “oxygen carrier particles”, “metal oxides”, “oxygen carrier” and “oxygen transfer material” are used interchangeably and include any reactive oxide components, alone or formed in combination with another material that serves as a physical carrier for the oxygen carrier that can be reversibly reduced and oxidized that are now known or that are developed in the future.